In a high volume production environment, such as a facility for fabricating semiconductor devices, the product usually requires processing at each of numerous stations or processing machines. The transporting or conveying of partially finished products, or WIP parts, is important element in achieving efficiency and quality in the overall manufacturing process. The ability to efficiently transport and track WIP parts is especially important in the manufacturing of semi-conductor devices, such as integrated circuit devices. Conveying and tracking WIP parts in a semi-conductor fabrication plant is particularly challenging, since as many as several hundred fabrication steps are required to complete the fabrication of a device, and these fabrication steps are carried out at a multiplicity of processing stations under extremely clean conditions, where human handling of the devices must be minimized. To complete the fabrication of an IC circuit, for example, various steps such as deposition, cleaning, ion implantation, etching and passivation must be completed before an IC chip can be packaged for shipment. Each of these fabrication steps must be performed by a different process machine, such as a chemical vapor deposition chamber, or an ion implantation chamber, etcher, etc. A partially processed semi-conductor wafer therefore must be conveyed between various work stations many times before it is completed. To achieve the goal of minimum handling by plant personnel, the IC chips are normally conveyed between workstations in a completely automated manner.
One of the automated methods of transporting wafers between work stations or process machines involves loading wafers into a container, sometimes referred to as a "pod", and then transporting the pod on an automated vehicle which rides on tracks established between work stations. In the case of the processing of semi-conductor wafers, the wafers are normally loaded into cassettes, which are placed into the pods. The cassette is loaded into the bottom end of the pod since the top is sealed off to reduce the possibility of contamination.
In order to identify and locate the various WIP parts contained in the pod, the pods are labeled with a tag positioned on the side of the pod. The tags can be read automatically by a tag reader that is normally mounted either along guard rails paralleling the guide tracks, or along an input conveyor of a storage facility, sometimes referred to as a "stocker", which is located at each of the processing stations. At each processing station, pods are periodically removed from the stocker for processing following which they are returned to the stocker until they are later automatically removed and conveyed to the next processing station where they are held in a different stocker. Although each pod tag is read by a tag reader as it enters the stocker, for a variety of reasons, pods cannot be positively located once they are within a stocker, and thus are subject to becoming lost within the tracking system. Lost pods in the tracking system may occur for a variety of reasons, including mechanical error, communication errors, or errors by operating personnel. In any event, the inability to positively track and locate all pods at any particular point in time materially reduces processing efficiency, and thus the device throughput of the fabrication facility.
Accordingly, there is a clear need in the art for an improved WIP parts tracking system which provides continuous positive tracking of the identity and location of the parts at all times, and which is not subject to mechanical or human error.